The invention relates to a tapping screw adapted to be screwed into a mounting surface of wood or the like while forming threads therein and more specifically a tapping screw having a structure for preventing cracks from being formed on the mounting surface when the screw is driven in.
FIG. 6 shows a prior art tapping screw which comprises a shank 11 having a tapered tip 12 and a head 13 formed integrally with the rear end of the shank and having a larger diameter than that of the shank 11. The latter is formed on its entire peripheral surface with threads 14.
The head 13 of this tapping screw has such a shape as to expand from the shank 11 to form a conical shape. In use, the screw is usually screwed into a mounting surface until its head 13 sinks below the surface.
But the tapping screw having such a structure has a drawback that because the threads 14 on the shank 11 have the same diameter and the same lead angle, when the shank is screwed in up to its rear end, a considerable amount of heat tends to be produced as the contact length of threads increases. This may cause the seizure of threads.
Also, when the screw is driven in until its head 13 sinks below the mounting surface, the mounting member A will be forcibly expanded radially by the head 13 as shown in FIG. 7. This may result in the development of cracks in the mounting member. Cracks are more likely to develop if the mounting member is a hard wood.
To prevent such cracks, it is necessary to reduce the stress which acts on the mounting member by reducing the size of the head. But because it is necessary to form a groove 15 for a screw driver in the head 13, its shape is limited and its size cannot be reduced below a given point.
One prior art tapping screw has its head tapered moderately into a conical shape in order to reduce changes in the stress which acts on the mounting member. But the eventual deformation resulting from the penetration of the head is the same as with other prior art screws. Thus if the mounting member is a hard wood, such a shape is of no advantage and cracks will develop as with other prior art screws.